linux-kernel - Re: [PATCH v17] exec: Fix dead-lock in de_thread with ptrace

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Message-ID: <aRRYzb2FxHzpKhms@redhat.com>
Date: Wed, 12 Nov 2025 10:52:13 +0100
From: Oleg Nesterov <oleg@...hat.com>
To: Bernd Edlinger <bernd.edlinger@...mail.de>
Cc: Christian Brauner <brauner@...nel.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
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Subject: Re: [PATCH v17] exec: Fix dead-lock in de_thread with ptrace_attach

On 11/11, Bernd Edlinger wrote:
>
> On 11/11/25 14:12, Oleg Nesterov wrote:
> > On 11/11, Bernd Edlinger wrote:
> >>
> >> Well when this is absolutely not acceptable then I would have to change
> >> all security engines to be aware of the current and the new credentials.
> >
> > Hmm... even if we find another way to avoid the deadlock? Say, the patches
> > I sent...
> >
>
> Maybe, but it looks almost too simple ;-)
>
>    164          sleep(2);
>    165          /* deadlock may happen here */
>    166          k = ptrace(PTRACE_ATTACH, thread2_tid, 0L, 0L);
>
> what happens if you change the test expectation here, that the
> ptrace may fail instead of succeed?
>
> What signals does the debugger receive after that point?
> Is the debugger notified that the debugged process continues,
> has the same PID, and is no longer ptraced?

Ah, but this is another thing... OK, you dislike 3/3 and I have to agree.

Yes, de_thread() silently untraces/reaps the old leader and after 3/3 debugger
can't rely on PTRACE_EVENT_EXIT, so unless the debugger has already attached to
all sub-threads (at least to execing thread) it looks as if the leader was just
untraced somehow.

OK, this is probably too bad, we need another solution...

Oleg.